If you are using react-native iOS then font-weight will work fine, but in react-native android you can't use font weight you should use particular font-weight of that font-family.

for example: Roboto: Roboto_thin 100 Roboto_thin 400 Roboto_thin 600 Roboto_thin 700 Roboto_thin 800 Roboto_thin 900

React-Native Example:

<Text style={{ color: '#FFFFFF, fontSize: 20, fontFamily: 'Roboto_thin 700', }}>